West Nile Virus

West Nile virus (WNV) is a newly emergent virus of the family Flaviviridae, found in both tropical and temperate regions. It mainly infects birds, but is also the cause of a number of conditions in humans, horses, and some other mammals. It is transmitted by bites of infected mosquitoes. This disease has recently become a concern for researchers because of its increased virulence and emergence in new geological locations, such as North America.

In the Old World, disease due to WN has been reported only in one Rock Dove (pigeon) found in Egypt. Fatal disease was produced experimentally in Hooded Crows. Many species of birds and mammals have antibodies against the virus, which indicates that infection is quite common. Disease appears to be very rare. However, in North America, the virus has proven to cause fatal disease in many species of native birds and a few species of mammals. Hundreds of thousands of American Crows have died of WN infection and other members of the crow family also appear highly susceptible to severe disease when they become infected. On the other hand, many species clearly survive infection with little or no evidence of disease and, in many species, some individuals become ill and may die when infected while many other infected individuals suffer no illness at all. A regularly up-dated list of animal species for which at least one individual is known to have died with West Nile virus infection is provided by the U.S. National Wildlife Health Laboratory.